The invention relates to a non-pneumatic, structurally supported tire. More particularly, the invention relates to a non-pneumatic tire that supports a load with its structural components and has pneumatic tire-like performance capabilities to serve as a replacement for pneumatic tires. The pneumatic tire has capabilities in load carrying, road shock absorption, and force transmission (accelerating, stopping, and steering) that make it the preferred choice for use on many vehicles, most notably, bicycles, motorcycles, automobiles, and trucks. Conventional non-pneumatic alternatives rely on compression of the ground-contacting portion for load support, for example, solid tires, spring tires and cushion tires, and lack the performance advantages of pneumatic tires. Accordingly, except in limited situations, known non-pneumatic tires have not found wide use as substitutes for pneumatic tires. A non-pneumatic tire having performance characteristics similar to those of pneumatic tires would overcome the various deficiencies in the art and would be a welcome improvement.
A structurally supported, non-pneumatic tire in accordance with the invention includes a reinforced annular band that supports the load on the tire and a plurality of web spokes that transmit in tension the load forces between the annular band and a wheel or hub.
According to an embodiment useful as a tire on a motor vehicle, a structurally supported tire includes a tread portion, a reinforced annular band radially inward of the tread portion, a plurality of web spokes extending transversely across and radially inward from the annular band toward a tire axis, and means for interconnecting the web spokes to a wheel or hub.
The structurally supported tire of the invention does not have a cavity for containing air under pressure, and accordingly, does not need to form a seal with the wheel rim to retain internal air pressure. The structurally supported tire does not, therefore, require a wheel as understood in the pneumatic tire art. For the purposes of the following description, the terms “wheel” and “hub” refer to any device or structure for supporting the tire and mounting to the vehicle axle, and are considered interchangeable herein.
The annular band comprises an elastomeric shear layer, at least a first membrane adhered to the radially inward extent of the elastomeric shear layer, and at least a second membrane adhered to the radially outward extent of the elastomeric shear layer. Each of the membranes has a longitudinal tensile modulus greater than the shear modulus of the shear layer, a ratio of a longitudinal tensile modulus of one of the membranes to the shear modulus of the shear layer is at least about 100:1, and a longitudinal compression modulus of the second membrane is at least equal to the longitudinal tensile modulus of the first membrane such that, under an externally applied load, the ground contacting tread portion deforms from essentially a circular shape to a shape conforming with the ground surface while maintaining an essentially constant length of the membranes. Relative displacement of the membranes occurs by shear in the shear layer. Preferably, the membranes comprise superposed layers of essentially inextensible cord reinforcements embedded in an elastomeric coating layer.
The annular band may further comprise an intermediate layer located within the shear layer and radially outward of the first membrane and radially inward of the second membrane. In a particular design example, the intermediate layer comprises cord reinforcements having a volume fraction between about 0.005 and 0.010, and the cord reinforcements may be oriented parallel to the tire circumferential direction.
The elastomeric shear layer is formed of a material, such as natural or synthetic rubber, polyurethane, foamed rubber and foamed polyurethane, segmented copolyesters and block co-polymers of nylon. Preferably, the shear layer material has a shear modulus of about 3 MPa to about 20 MPa. The annular band has the ability to bend from a normal circular shape while under load to conform to a contact surface, such as a road surface.
The web spokes act in tension to transmit load forces between the wheel and the annular band, thus, among other functions, supporting the mass of a vehicle. Support forces are generated by tension in the web spokes not connected to the ground-contacting portion of the annular band. The wheel or hub can be said to hang from the upper portion of the tire. Preferably, the web spokes have a high effective radial stiffness in tension and a low effective radial stiffness in compression. The low stiffness in compression allows the web spokes attached to the ground-contacting portion of the annular band to bend for absorbing road shocks and for better conforming the annular band to the irregularities in the road surface.
To facilitate the bending of the web spokes of the ground contacting portion of the tread, the spokes can be curved. Alternatively, the web spokes can be pre-stressed during molding to bend in a particular direction.